1. Field of the Invention
The present invention relates to an underwater sounding apparatus capable of eliminating the influence of propeller noise.
2. Description of the Prior Art
FIG. 20 illustrates how an underwater sounding apparatus obtains information on an underwater situation, such as distribution of fish schools. Referring to FIG. 20, designated by the numeral 80 is an underwater sounding apparatus installed on a vessel 81, designated by the numeral 82 is an acoustic transducer unit of the underwater sounding apparatus 80, designated by the numeral 83 is a propeller of the vessel 81 and designated by the numeral 84 is a water surface. Also, designated by the letters Bt is a transmitting beam emitted from the transducer unit 82, designated by the letters Br is a receiving beam for receiving echo signals returned from underwater objects which reflect part of energy of the transmitting beam Bt and designated by the letter α is a tilt angle of the transmitting beam Bt.
The transmitting beam Bt is transmitted underwater simultaneously in all directions around the transducer unit 82, typically forming an umbrellalike beam pattern directed obliquely downward by as much as the tilt angle α. The receiving beam Br, on the other hand, is a rotating beam having specific directivity. The receiving beam Br is electronically steered around the transducer unit 82 by successively switching vibrating elements thereof. The underwater sounding apparatus 80 obtains information about fish schools and other underwater objects by processing and analyzing the echo signals received by the receiving beam Br.
Japanese Laid-open Patent Publication Nos. 2001-343450 and 202370 disclose methods of controlling the tilt angle of a transmitting beam formed by an acoustic transducer unit having a plurality of vibrating elements arranged in multiple rows and columns on a cylindrical outer surface of a transducer body as well as methods of forming a receiving beam. Also, Japanese Laid-open Patent Publication No. 2003-084060 discloses an electrical block diagram of a scanning sonar which is an example of the underwater sounding apparatus.
Typically, the transducer unit 82 transmits and receives ultrasonic sounding waves (acoustic waves) while the vessel 81 is moving so that sound waves produced by the revolving propeller 83 act as noise superimposed on the echo signals.
FIG. 21 is a diagram showing an example of false images (false echoes) 90–92 presented on a display unit of the underwater sounding apparatus 80 due to propeller noise when the tilt angle α of the transmitting beam Bt and the sounding range of the underwater sounding apparatus 80 are set to 0° and 400 m, respectively. Here, the false images refer to echoes presented on the display unit due to signals (e.g., propeller noise) other than true echo signals. As illustrated in FIG. 21, the vessel 81 is located at a screen center (sweep center) and the heading of the vessel 81 lies on a line connecting the sweep center (in head-up presentation).
In the example of FIG. 21, the false echo 90 painted in a darker shade is located behind the vessel 81 while the false echoes 91, 92 painted in a lighter shade lie on left and right sides of the vessel 81 slightly behind a line drawn athwartships. A reason why the false echoes 91, 92 occur obliquely rearward is presumably that left and right side lobes of the receiving beam Br lie approximately in a propeller noise direction when the receiving beam Br is oriented in the directions of these two false echoes 91, 92. The false echoes 90–92 overlying true echoes could obscure or totally hide away echo signals of real targets, such as fish schools.
Generally, it is difficult for a user to distinguish true echoes from the false echoes 91, 92 presented obliquely rearward. Thus, the underwater sounding apparatus 80 conventionally has a problem that its detection performance is deteriorated by such false echoes caused by the propeller noise. The influence of the propeller noise is, by its nature, more significant when the tilt angle α of the transmitting beam Bt is small, and when the distance between the transducer unit 82 and the propeller 83 is small (i.e., on smaller vessels).
An approach to the solution of the aforementioned propeller noise problem is disclosed in Japanese Laid-open Patent Publication No. 1991-017583, in which an underwater sounding apparatus has, in addition to an ordinary transducer unit directed vertically downward for target sounding, a hollow cylinder-shaped transducer unit surrounding the sounding transducer unit and dedicated exclusively to receiving propeller noise. According to this prior art approach, a propeller noise signal received by the dedicated transducer unit is delayed and its gain is controlled such that the received propeller noise signal matches in amplitude and timing (phase) with the propeller noise contained in echo signals received by the sounding transducer unit. Then, the propeller noise is canceled out by subtracting the delayed and gain-controlled propeller noise signal from the echo signals received by the sounding transducer unit.
The approach of Japanese Laid-open Patent Publication No. 1991-017583 requires the provision of the dedicated propeller noise receiving transducer unit surrounding the sounding transducer unit. When this approach is applied to an underwater sounding apparatus employing a plurality of vibrating elements arranged in multiple rows and columns on a cylindrical outer surface of a transducer body, for example, there arises a problem that the propeller noise receiving transducer unit intervenes in propagation paths of transmitting and receiving beams, making it impossible to detect underwater target information. In addition, the need for the provision of the dedicated propeller noise receiving transducer unit results in an overall cost increase due to an increase in the number of components.